This invention relates to a method and apparatus for measuring the wall thickness of transparent objects, such as glass tubes.
In accordance with well known prior art techniques, the wall thickness of transparent objects (i.e., glass tubes) have been determined using a method wherein a narrow band of light is projected onto a surface of a transparent object at an angle other than 90.degree.; and the beams reflected by the adjacent boundary surface of the object and that boundary surface thereof which is spaced behind that adjacent boundary surface by the distance to be measured are focussed onto the image plane of a receiver. A movable deflecting element is provided in the optical path between the light source and the image plane of the receiver. The lines of intersection between the band of light and the boundary surfaces of the layer to be measured (which lines of intersection are represented by bands of light on the image plane of the receiver), are moved by the deflecting element in the direction in which they are spaced apart. Signals are then generated by the receiver in response to the appearance of first and second bands of light at a predetermined location on the image plane of the receiver; and the interval of time between the signals is measured to determine the wall thickness of the transparent object.
Methods of the general type described above are known in the art. An example of such a prior art method for measuring the wall thickness of transparent objects is disclosed in Published German Application No. 18 03 285.
Those light beams which can be produced by presently available light sources, and the associated image-forming means, are very sharply defined and permit a projection of images consisting of narrow bands of light of high sharpness and high intensity. However, the reflection on the adjacent boundary surface of the object to be examined, and on that boundary surface thereof which is disposed behind the adjacent boundary surface at the distance to be measured, results in the formation of reflected beams of reduced sharpness and of irregular shape because the boundary surfaces of the object to be examined may have corresponding irregularities. As a result, it is difficult to achieve an accurate measurement. For this reason, the images which are formed by the narrow bands of light in the image plane of the receiver have reduced sharpness, variations in strength and differences in intensity. These problems are particularly prevalent in beams which have been reflected by the adjacent boundary surface of the object; and by that boundary surface thereof which is spaced behind that adjacent surface by the distance to be measured.
It will be appreciated that the above-described reduced sharpness, strength variation and intensity difference of the reflected beams, on the projected image of the narrow bands of light on the receiver adversely affect the accuracy of the measurement. In order to avoid such adverse effects, a measurement is taken of the interval of time from the occurrence of the center or the peak value of a first output signal generated by the receiver in response to the projection of a first image of the narrow band of light on the receiver, and the occurrence of the center or peak value of a second signal produced by the receiver in response to the projection of a second reflected image of the narrow band of light.
The above described measuring technique is disclosed in aforementioned Published German Application No. 18 03 285 wherein a mean or average of two measured intervals of times is calculated, the first of which is the interval of time between the times at which the first and second signals rise above a predetermined threshold value; and the second interval of time is the interval between the times at which the first and second output signals of the receiver decrease below that threshold value.
In practice, however, the generation of the signals which represent the intervals of time in the prior art method (of German Application No. 18 03 285) requires a multiple differentiation of signals from the receivers. This may give rise to certain problems, particularly if the images of the narrow band formed on the receiver by projection or otherwise are blurred or streaky as a result of irregularities of the reflecting surfaces of the object to be examined leading to secondary peaks appearing in the resulting output signals produced by the receiver. Moreover, the reflection of the beam derived from the image that has been projected on the second reflecting surface, which is disposed behind the adjacent boundary surface, will result in a considerable loss in intensity of the second reflected image formed on the receiver by the narrow band of light. As a result, the output signal of the receiver will be correspondingly weakened.
From German Patent Specification No. 23 25 457, it is also known to provide, at the location of the receiver, a linear array of detectors, which are spaced a preselected distance from each other; and to direct a beam of light on the object to be examined so that the beam of light is reflected by the adjacent boundary surface of the article and by that boundary surface thereof which is spaced below the adjacent boundary surface by the distance to be measured. The reflected beams are incident on different detectors, and the distance between the detectors in the array can be determined by consecutively sampling the detectors of the array.
In that prior art method (German Specification No. 23 23457), the accuracy of measurement is limited by the pitch of the detectors of the array and by the resolution provided by them; and it is difficult to provide for a plurality of distinct measuring ranges.